The most important lead ore is galena, which consists primarily of lead sulphide. Production of lead from such ores typically involves a froth flotation step to form a lead sulphide containing concentrate. The lead sulphide containing concentrate typically includes lead sulphide, zinc sulphide, iron sulphide, silica and calcium oxide. The concentrate is subsequently smelted to produce lead metal.
Conventional lead smelting plants include a sinter plant. The concentrate passes through the sinter plant prior to the smelting step in a blast furnace. In the sinter plant, the concentrate is burned or roasted to produce an off gas containing sulphur dioxide and sintered product containing lead oxide, silica and other oxides. The sinter plant oxidizes the concentrate and removes the bulk of the sulphur from the concentrate.
Typical sinter plants have a moving grate on which the concentrate rests. The moving grate moves over a number of wind boxes, through which a current of air blows upwards. The sinter plant requires special feed control, particularly of particle size and moisture content, in order to ensure proper operation of the sinter plant. Very large sinter recycle ratios are also required in order to control the amount of heat generated in the sinter plant. It is important to control the operation of the sinter plant in order to avoid the formation of any lead metal in the sinter plant, as this would block the moving grate within the sinter plate.
In the sinter plant, the sulphide species are largely converted to oxides and fine powders are agglomerated into lumps. The agglomerated particles may be broken up to a size convenient for use in the downstream blast furnace. The sinter plant gases are routed to gas cleaning equipment for recovery of any fumes and for the removal of sulphur containing gases to form sulphuric acid.
The sinter leaving a sinter plant is subsequently used as a feed to a lead smelting blast furnace. The sinter is mixed with a carbonaceous material (typically coke) and a flux (such as limestone) and fed into the top of a blast furnace. In the blast furnace, air is injected through tuyeres located towards the bottom of the blast furnace. As the air passes upwardly through the furnace, it causes combustion of some of the coke to supply energy for the smelting process. The presence of coke ensures that a reducing atmosphere is largely maintained within the reactive zones of the furnace, thereby reducing the lead oxide in the sinter to lead metal. Lead metal is tapped off from the bottom of the furnace and either cast into ingots or collected in ladles for transferring to a lead refining process. The lead metal that is collected from the blast furnace is conventionally referred to as lead bullion, because that lead metal acts as a collector for any precious metals in the concentrate.
The above described conventional process for producing lead (incorporating a sintering plant and a blast furnace) is used to recover approximately 80% of worldwide lead production.
Other processes for recovering lead from sulphide ores and concentrates have also been developed. These processes include Kivcet process, the QSL process and the ISASMELT process.
The ISASMELT process utilises gas injection into melts via a top entry submerged lance. Injection of gases via the top entry submerged lance produces a very turbulent bath in which high intensity smelting or reduction reactions take place. In the ISASMELT process, a two stage process may be utilised. In the two stage process, lead concentrate is added directly to a molten slag bath in a smelting furnace. This produces a lead containing slag, which is transferred to a second furnace in which that lead containing slag is reduced to form lead bullion. Both furnaces use top entry submerged lances for injection of gases.
The ISASMELT process can also be used to directly reduce some of the concentrate added to the smelting furnace to lead bullion. Typically, concentrates containing high levels of lead, such as between 55% to 80%, but more preferably between 60% to 75% have been processed in this manner, although concentrates having lead concentrations outside this range may also be processed using direct smelting.
It is an object of the present invention to provide an alternative lead smelting method and apparatus for producing lead from lead sulphide containing materials.